September 6, 2013

NASA Scientists Discover Cool Brown Dwarfs That Are Still Pretty Warm

Two years ago NASA scientists revealed a new class of stars, dubbed ‘Y dwarfs,' thought to be about as warm as the human body. However, newly published research indicates that these cold stars are probably a bit warmer than the boiling point of water.

These stars are believed to have a mass about 5 to 20 times that of Jupiter. Unlike our sun, these types of brown dwarf stars derive their energy via gravitational contraction, a process that depends directly on their mass.

"If one of these objects was found orbiting a star, there is a good chance that it would be called a planet," said Trent Dupuy, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics and co-author of the new study, which was published in the journal Science Express. Because they most likely formed on their own and are not sitting in a proto-planetary disk, astronomers still refer to these objects as brown dwarfs even if they have "planetary mass."

Getting accurate information of this class of stars can be difficult due to their small size, remoteness and the fact that they only emit light in the infrared spectrum. Using NASA's Spitzer Space Telescope, the team examined Y dwarfs between 20 and 50 light years away.

"We wanted to find out if they were colder, fainter, and nearby or if they were warmer, brighter, and more distant," Dupuy said.

To calculate these distances, the team measured their perceived change in position against a backdrop of stars over time, also known as parallax. As the Spitzer Space Telescope orbits the Sun, its perspective changes – giving nearby objects the appearance of slightly shifting back and forth. The same effect can be experienced if you hold an object in front of your face and close one eye and then the other. The result is the appearance of the object changing positions.

"To be able to determine accurate distances, our measurements had to be the same precision as knowing the position of a firefly to within 1 inch from 200 miles away," said study co-author Adam Kraus, professor at the University of Texas at Austin.

The study researchers noted that their findings could be used by those who study cool, planet-like atmospheres. Unlike warmer objects, the visible properties of these objects don't seem to be strongly linked with their temperature. This indicates that other factors, such as convective mixing, strongly affect the chemistry at the celestial object’s exterior. The study researchers said they also discovered evidence for disappearing alkali elements that are probably getting incorporated into toxic clouds.

The latest study expanded on the findings of NASA’s Wide-field Infrared Survey Explorer (WISE), which identified six Y-class brown dwarfs two years ago. Since its launch in 2009, WISE has revealed hundreds of new brown dwarfs, including 14 that are considered to be cool Y dwarfs.

Earlier this year, NASA scientists examined the atmosphere of one particular brown dwarf using the Hubble and Spitzer space telescopes. They were able to detect wind-driven clouds about the size of planets.